ELECTRICAL METHODS 441 



and by Held measurements in which average resistivities of subsurface 

 materials and outcrops are measured in place. 



The electrical resistivities of earth materials vary within very wide 

 limits. For example, the resistivity of certain metallic elements is about 

 10~^ ohm-centimeters and the resistivity of certain igneous and metamor- 

 phic rocks is greater than 10'^ ohm-centimeters. 



The accompanying tables list some resistivity values obtained in field 

 explorations in the United States and Canada and resistivity values for 

 laboratory samples. Figure 265 is a graphical correlation of effective 

 resistivities of earth materials of different geological periods. 



TABLE 15 



RESISTIVITIES OF MATERIALS FOUND IN THE OUTER 

 CRUST OF THE EARTH 



Resistivity 

 Igneous and Metamorphic Rocks ohm-cm 



Basalt 2xl0« 



Crystalline rock of normal physical 



character (igneous, gneiss, schist) . . . 2x10* - 2x10" 



Diabase 2x10=* - 2x10" 



Diorite 5x10" 



Gabbro 1x10* - 1.5x10" 



Gneiss 2x10* - 3.4x10" 



Granite 3x10* ->10" 



Lava 1.2x10* - 5x10" 



Marble 1x10* -1x10' 



Meteoric iron 1x10"" - 3x10-' 



Porphyry 6x10' - 1.5x10" 



Quartzite 1x10' - 2x10' 



Schist 5x10' - 1x10" 



Serpentine 2x10* - 3x10* 



Syenite 1x10* - 10' 



Trachyte 1x10' - 1x10' 



Trap Rock 1.5x10* - 3x10" 



Resistivity 

 Sedimentary Rocks ohm-cm 



Alluvium and Silt 2.5x10' - 1.5xl0'^ 



Clay-Shales 4x10"= - 9x10* 



Clay 5x10= - 1.5x10" 



Glacial sediments 8x10"' - 9.5x10" 



Conglomerate 2.5x10' - 1.5x10" 



Consolidated sedimentary rocks 

 (slates, shales, sandstones, lime- 

 stones, etc.) 1x10' - 5x10* 



Graywacke 2x10" - 10" 



Limestone . .'. 6x10' -5x10' 



Loams 1x10' - 4.5x10* 



Marls 0.5x10^ - 7x10' 



Sand 9.5x10^ - 5x10" 



Sandstone 3x10' -1x10' 



Shales 8x10=* - 1x10" 



